In general, mobile communication systems have been developed to provide communication while securing mobility of users. With the rapid development of technologies, the mobile communication systems have reached a stage of providing high-speed data communication services as well as voice communication.
Currently, a standardization operation from a 3rd Generation Partnership Project (3GPP) system to a Long Term Evolution (LTE) system is being progressed as one of next generation mobile communication systems. An LTE system corresponds to a technology which implements a high-speed packet based communication having a transmission rate of up to 100 Mbps which is higher than a currently provided data transmission rate, and standardization thereof has almost been completed.
Recently, discussions about an advanced LTE communication system (LTE-Advanced, hereinafter, referred to as LTE-A) which improves the transmission rate by combining several new technologies to the LTE communication system have taken place. Carrier aggregation may be a representative technology among the newly introduced technologies. The carrier aggregation means that one UE transmits and receives data uses a plurality of forward carriers and a plurality of backward carriers unlike that the UE uses only one forward carrier and one backward carrier in the prior art.
Currently, in LTE-A, only intra-eNB carrier aggregation is defined. This leads to a result of reducing the applicability of a carrier aggregation function, in particular, in a scenario of overlapping and operating a plurality of pico-cells with one macro cell, a problem in which the macro cell and the pico-cells are not aggregated may occur. 3GPP has proceeded a study in the name of “Small cell enhancement” in order to solve these problems in Release 12. As an aspect of the study, a technology of an inter-eNB carrier aggregation or dual connectivity between heterogeneous eNBs, which supports a high data transmission rate to one UE by combining serving cells subordinate to another eNB has been looked at as a representative (hereinafter, dual connectivity is the general term for inter-eNB carrier aggregation or dual connectivity between heterogeneous eNBs). Fields such as other mobility resources have been actively discussed, but a carrier aggregation technology supported in only the existing eNB is possible between a macro eNB and a pico-cell or small cell eNB so that it is expected to have a large impact on future communication technology. As data use of a smart phone increases rapidly in the future, it is predicted that small cells will increase exponentially, and small cell eNBs which can independently accept a UE together with a small cell configuration using an existing Remote Radio Heads (RRH) will occupy a large part of the market. Accordingly, when the UE receives data transmission by connecting to the small cell, the UE can receive different kinds of data from the macro eNB simultaneously.